Recent Advances in Silica-Based Materials for the Removal of

Review pubs.acs.org/jced

Recent Advances in Silica-Based Materials for the Removal of Hexavalent Chromium: A Review Manish Kumar Dinker and Prashant Shripad Kulkarni* Energy and Environment Laboratory, Department of Applied Chemistry, Defence Institute of Advanced Technology (DU), Girinagar, Pune, 411025, India ABSTRACT: Hexavalent chromium [Cr(VI)], one of the most toxic contaminants, is released in the environment due to various anthropogenic activities. Exposure of Cr(VI) can pose a serious threat to the public health as well as flora and fauna. Effective treatment of Cr(VI) is, therefore, very essential from safety, health, and environment points of view. The present review focuses on the development of silica-based materials for the adsorption of Cr(VI) from wastewater. After discussing toxicity issues and general removal methods of Cr(VI), the importance of silica materials are highlighted. The silica has different shapes, sizes, surface areas, and pore diameters and, hence, can play a vital role in designing the adsorbent. They can be modified into organic, inorganic, polymeric, biological, and ionic liquid based materials. Therefore, they are broadly classified into these five categories. The adsorption isotherms and kinetics of these materials for Cr(VI) are discussed and compared with each other. Future prospects based on the findings of the review article are summarized in the end which mainly emphasizes the importance of biosorbents and ionic liquid immobilized silica materials for the treatment of Cr(VI).

1. INTRODUCTION The existence of metal oxyanions, such as As(V), Cr(VI), Mo(VI), and Se(VI) in groundwater is one of the most serious environmental problems today. These toxic contaminants are coming in the ecosystem because of industrial and agricultural processes as well as mining activities and, hence, possess a serious threat to the human beings as well as flora and fauna. Among the toxic metal oxyanions, chromium is a common contaminant in the surface water coming from various industrial activities like paint industry, textile dyeing, leather tanning, metal finishing, and electroplating.1 It is found in the environment both as trivalent [Cr(III)] and hexavalent [Cr(VI)] form, although toxicity of hexavalent form is five hundred times greater than the trivalent form.2 It can exist in the solution as various species, which is entirely dependent on the pH of the solution and its concentration. Figure 1 depicts the relative distribution of Cr(VI) species in water as a function of pH. It shows that chromic acid (H2CrO4) occur when pH is less than 1. Further, from acidic pH 1 to the neutral pH 7, the hydrogen chromate ion (HCrO4−) exist, whereas, above the neutral pH, only chromate ions (CrO42−) exist in the solution. The dichromate ions occur when the Cr(VI) concentration goes beyond 1 g·L−1.3,4 It is evident that the harmful effects of chromium are mainly due to Cr(VI) because of its mobility in the environment as well as the ability to oxidize other species. Therefore, it has low, acute, and chronic toxicity to humans at high doses.5,7 The direct exposure to Cr(VI) causes irritation of eyes, allergic reactions, dermatitis, asthma, or even skin burn, and its presence in the body leads to the lung and kidney cancer and bronchogenic carcinoma. Therefore, various agencies such as WHO and USEPA have given a tolerable limit of 0.05 mg·L−1 for dissolved Cr(VI) in drinking water, and that for total © XXXX American Chemical Society

Figure 1. Relative distribution of Cr(VI) species in water as a function of pH and Cr(VI) concentration (adapted from ref 3).

chromium (all form of chromium) is 2 mg·L−1.8 Hence, it is necessary to treat Cr(VI) originating from various effluents to meet the stringent discharge standards. Several methods have been applied to reduce the harmful effects of Cr(VI) such as chemical extraction,9,10 reduction− precipitation,11,12 reverse osmosis,13,14 electrokinetic remediation,15,16 bioleaching process,17 and ion exchange. 18,19 Adsorption is a fundamental and economically viable process for the treatment of Cr(VI) from wastewater because of its several advantages such as high removal efficiency, low energy demand, less chemical investment, and reusability. Adsorption Received: March 28, 2015 Accepted: August 10, 2015

A

DOI: 10.1021/acs.jced.5b00292 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Review

ordered mesoporous silicas such as SBA-1, SBA-15, MCM-41, and MCM-48 with large surface area containing high amount of silanol groups (slightly acidic) and high porosity.49−52 The ordered mesoporous silicas are further modified by immobilization of various functional groups (particle surface as well as the pore wall surface) to form organic−inorganic hybrid materials.53−55 In the adsorption process, it is very important to consider the factors which influence the capacity of the modified silicas for the removal of Cr(VI) from wastewater. The first important factor is the structural texture and characteristics of the silica materials such as surface area, porosity, and the availability of surface silanols. For example, Jal et al. reported that silica gels because of having high surface area, high mass exchange characteristics, and availability of high siliceous surfaces, are the mostly utilized substrate to be modified for the removal of toxic metal ions.56 In case of mesoporous silica (e.g., SBA-15 and MCM-41), Galarneau et al. demonstrated that bimodal structural of SBA-15 having straight hexagonal mesopore channels attached to each other by thick walled micropores provides high mechanical stability than the MCM-41.57 Moreover, because of having distinct mesopores channels, both of them (SBA-15 and MCM-41) have shown different adsorption properties upon their surface modifications.58 Hence, surface modification of silica materials is the second important factor to be considered to prepare adsorbents. Hoffman and co-workers described two main processes for silica surface modification by employing organic groups.59 The first process is known as the postsynthesis grafting where the modification of the mesoporous silica is carried out by the functionalization of the suitable organosilane with the surface silanols of the presynthesized mesoporous silica. Another method directly approaches to the hydrolysis process where the organic groups are added to the silica precursor (TEOS) in the presence of surfactant which further via co-condensation route resulted into the formation of organically modified mesoporous silica. This procedure is known as direct or co-condensation method (one-pot synthesis). Some authors have done comparative studies between the direct synthesis and postsynthesis grafting methods with respect to the advantages and disadvantages of them.60−63 Moreover, Yoshitake has reported a process known as semidirect synthesis where the organosilane are incorporated to the hydrolyzed TEOS gel before the hydrothermal process, which resulted into the functionalized ordered mesoporous silica exposing more functional groups than in direct synthesis.64 It should be noted that the surface modifications of the silica materials is not only bounded to the exploitation of organic groups, but it has also been moderately practiced the utilization of inorganic groups, polymeric compounds, biological materials and recently, the ionic liquids functionalization on the silica materials, which can be worked as oppositely charged (cationic) moieties to interact with the metal oxyanions by the mean of electrostatic attraction.65−69 Additionally, their loading on the silica materials is equally important, since they relate to the number of binding sites obtained for the adsorption of metal oxyanions.70 Moreover, functionalization of these groups has a huge impact on the structural parameters of the silica materials such as their specific surface areas, pore volumes and pore sizes are found to be altered after surface modification.71−74 Overall, functionalized silica materials proved to be an effective adsorbent for sequestering toxic metal ions, particularly Cr(VI) from wastewater. Therefore, in the following discussion, an

of Cr(VI) from the aqueous streams to the surface of the adsorbent occur due the electrostatic interaction or via a chemical reaction. The adsorption process is continued until the Cr(VI) ions present in the solution achieve equilibrium with the adsorbent. The adsorption increases with an increase in the concentration of equilibrium analyte until a constant value is attained.20,21 Therefore, it is a parameter which shows the capacity of an adsorbent for Cr(VI) adsorption. Further, the process can be elaborated on the basis of two prominent models, adsorption isotherm and adsorption kinetics. Adsorption isotherm consists of two important parameters, Langmuir and Freundlich isotherms. Langmuir model is a single component (monolayer) adsorption model which relates to the adsorption process at a particular homogeneous binding site on the adsorbent which comes to end when the entire binding site is occupied.22,23 The Freundlich isotherm model is based on the adsorption of metal ions on the heterogeneous binding sites which may or may not be equivalent.24 On the contrary, adsorption kinetics is another crucial parameter, i.e., pseudo first and pseudo second order kinetics, to examine the mechanism of adsorption process in order to analyze the Cr(VI) uptake rate. Pseudo first order kinetics is based on the rate of adsorption which is proportional to the number of free binding sites of the adsorbent,25 whereas pseudo second order kinetics is directly proportional to the number of unoccupied binding sites.26 Several investigations are carried out for the adsorption of Cr(VI) using adsorbents such as activated carbons,27,28 zeolites,29−31 clays,32,33 nanomagnetic particles,34 and low-cost biosorbents.35 However, these adsorbents suffer from low adsorption capacities and selectivities because of having less porosity, low surface area, and lack of functional groups.36 For example, in the case of zeolites, sometimes their surface modifications with organic and inorganic groups led to the surface deactivation, which resulted in their inadequacy toward Cr(VI) adsorption.37 Therefore, there was a need to develop adsorbents with high porosity, large surface area, and high functionalities; hence, modified silica materials are largely investigated for the removal of Cr(VI) from wastewater.38 An enormous amount of literature is available in this area which needs critical evaluation and proper documentation. In view of this, a review based on the adsorption of Cr(VI) using silica based materials was undertaken for the investigation.

2. IMPORTANCE OF SILICA-BASED MATERIALS AS ADSORBENTS Silica is an inorganic solid, made up of three-dimensional network of tetrahedral vertices (four O atom around the Siatom) joined by a common oxygen atom (bridged), which gives a structure of porous material having a large surface area. It possesses attractive physical and chemical properties such as water stability (nonswelling), thermal stability (up to 1500 °C), and good mechanical strength. Moreover, the presence of silanol groups (Si−OH) on the surface makes silica a better solid support for the immobilization of various organic and inorganic groups.39 The synthesis of silica materials such as amorphous silica, mesoporous silica, fumed silica, and silica gels have been widely practiced and reviewed.40−46 The important ones, mesoporous silicas, can be prepared by the hydrolysis of alkoxysilane precursor (tetramethoxy- or tetraethoxysilane) in the presence of suitable surfactant (template) and catalyst, to form condensed polymerized network of siloxanes (Si−O−Si linkages).47,48 The sol−gel chemistry leads to the synthesis of B



Review

mmol·g−1) than MCM-41 (0.993 mmol·g−1). It could be attributed to the difference in the framework structure and pore wall surfaces of SBA-1 and MCM-41. More aminosilanes gets functionalized over the surface of SBA-15, whereas pore mouth of MCM-41 gets blocked, and the functional group remain under utilized on the adsorbent. Therefore, in SBA-1, all of the amino groups work as adsorption site for the chromate ions.77 A similar study of the utilization of mono, di, and triamino groups were reported when the functionalized organic group trialkoxysilane (RSi(OR)3) (where R = −N, −NN, −NNN, and − S groups) were placed in the simple silica matrices. These neat porous silica materials were synthesized by using TEOS (tetraethyl orthosilicate) as a precursor, and dodecyl amine was used as a structure-directing agent.78 The adsorption capacity (qe) of neat porous silica materials (0.1, 0.2, and 0.25 mmol·g−1 for N, NN, and NNN−S adsorbent) was found to be much lower than SBA-1 and MCM-41, as given in Table 1. It may be due to the less ordered pore walls and lower surface area in the former. Interestingly, the structure directing agents have shown further significant role in the synthesis of functionalized silicas. For example, the formation of functionalized silica powder by direct co-condensation of N-[3-(trimethoxysilyl)propyl]-ethylenediamine (TMSEDA) and trimethoxy orthosilicate (TMOS) with the trimesic acid (the benzene-1,3,5-tricarboxylic acid, TMA) as a structure-directing agent. It resulted in the generation of homogeneously distributed N atoms and amide linkages [−CO−NH−] upon the silica network.79 This nanohybrid powder got protonated in the acidic solution (pH 2.0) and showed maximum removal capacity (qm) of 0.74 mmol·g−1 for Cr(VI). The adsorption data was well-fitted with the Freundlich model (as depicted in Table 1). The AAPTS functionalized MCM-48 and fumed silica adsorbents80 have shown adsorption capacities lower than the adsorbent SBA-1.76 It is because fumed silica and MCM-48 are less porous which did not allow enough functionalizations. Thus, it showed low adsorption capabilities as given in Table 1. However, MCM-48 with AAPTS has shown better results as compared to fumed silica with AAPTS that may be due to the larger surface area in the former. Recently, mesoporous silica MCM-41 was functionalized by using monoamino [3-aminopropyl trimethoxysilane] (APTMS) and triamino [N-(3-trimethoxysilyl propyl) diethylenetriamine] (DETA) group, and the results were compared. It was observed that the grafting of AP-MCM-41 created simple chelation on the silica surface without any steric hindrance, due to which it yielded high adsorption of Cr(VI) than DETA-MCM-41.75 Notably, 100% adsorption efficiency was shown by the APMCM-41 and 85% by DETA-MCM-41 at similar experimental conditions, and the reusability of both the adsorbent was found to be quite different, too. Lam et al. also showed successful removal of Cr(VI) in the presence of Co, Cu, Ni, Zn, Ag, Pb, Hg, and Cd ions by using adsorbent AP/NH2-MCM-41.81 Surprisingly, Li et al. have reported highest adsorption capacity of 2.28 mmol·g−1 while using adsorbent NN-silica.82 The efficiency of aliphatic amines modified silica was compared with the aromatic amines modified silica by the group of Li and co-workers.83,84 They functionalized ordered mesoporous SBA-15 by aminopropyl, imidazole, and triazole by the facile one or two-step postgrafting method and successfully prepared adsorbents, AP-SBA-15, Im-SBA-15, and Tri-SBA-15. Later, these adsorbents were characterized and used for the removal of Cr(VI) from aqueous solutions. Considering the

attempt has been made to review the investigation of Cr(VI) adsorption using silica based (organic, inorganic, polymer, biological, and ionic liquid) materials.

3. ORGANICALLY MODIFIED SILICA MATERIALS Numerous works have been done on the basis of organic groups functionalized silica materials for the removal of Cr(VI). Especially, nitrogen-containing organic groups has been widely studied with silica, for this application. The processes such as postgrafting and direct synthesis are involved in the preparation. As discussed earlier, silica not only provides solid support to the organic groups but also enhances the adsorption of metal ions. The functionalized groups get fixed in the pores of porous silica and have shown high loading of organic contents (C, H, N atoms). The loading resulted into the varied mesoporous structure of the silica materials. The most valuable nitrogen containing functional group is aliphatic amines. Figure 2 shows the typical grafting of amino group, 3-aminopropyl

Figure 2. A diagrammatic representation of organically modified silica materials depicting (a) mesoporous silica modified by grafting 3aminopropyl trimethoxysilane (APTMS) on the surface silanols and (b) the amine group gets protonated in the acidic medium, which resulted in the attachment of anionic Cr(VI) by electrostatic attraction.

trimethoxysilane (APTMS) with silanol of mesoporous silica, subsequently resulting in the formation of adsorbent, N-silica.75 The adsorbent, in acidic medium, undergoes electrostatic attraction upon contact with wastewater containing Cr(VI). The amino groups due to their capability of converting into protonated ions in the acidic solutions have shown an increase in the removal capacity of Cr(VI) with an increase in the number of nitrogen atoms. A similar finding was observed when the mesoporous silicas SBA-1 and MCM-41 were functionalized with mono (−N), di (−NN), and triamino (−NNN) groups. As shown in Table 1, the triamino groups are far better with having three times higher possibility of ligating with chromate ions than that of diamino group to neutralize the CrO42− charges.76 However, the SBA-1 silica modified with triamino group, AAATS, removed more chromate ions (1.81 C


D

porous silica

MCM-48

MCM-41

SBA-15

SBA-1

types of silica

CTAB TMPA with MCM-41-C DTAB TMPA with MCM-41-D ETAB TMPA with MCM-41-C AAPTS AAATS CTAB MTMS APTMS AAPTS AAATS TMSEDA

neat SBA-1 APTMS AAPTS AAATS neat SBA-15 APTES 1,2,4-triazole imidazole neat SBA-15 imidazole neat SBA-15 MP neat MCM-41 APTMS AAPTS AAATS neat MCM-41 APTMS neat MCM-41 APTES CTAB CTAB

modified by

NH2-MCM-41 calcined MCM-41 MCM-41-SH/ SO3H MCM-41-C N-MCM-41-C

2.53

2.1 1.8 1.6 1.1 2.99

MCM-41-E N-MCM-41-E

0 2.1 NN-MCM-48 NNN-MCM-48 calcined MCM-48 S−S N−S NN−S NNN−S

MCM-41-D N-MCM-41-D

0 1.8

0 1.6

0.90

AP-MCM-41

1.99

MP-SBA-15

0.89 N-MCM-41 NN-MCM-41 NNN-MCM-41

Im-SBA-15

1.84

1.46 2.76 3.53

AP-SBA-15 tri-SBA-15 Im-SBA-15

N-SBA-1 NN-SBA-1 NNN-SBA-1

3.1 1.7 1.8

1.90 3.33 4.39

573 888 65 220 314

1278 575

1120 504

1045 580

1221 689 606 126 873 524 435 362 873 362 656 342 1283 1037 586 481 760 425 1070 750 461 860

m2·g−1

mmol·g−1 adsorbent name

surface area

loading of ligand (L0)a

3.1

3.1 2.7

3.3 2.9

3.1 2.8

3.02 2.66 2.48 2.40 6.5 5.6 5.8 5.6 6.5 5.6 6.02 5.03 2.90 2.80 2.63 2.52 6.7 6.4 3.09 2.92 4.13 2.0

nm

pore size

0.41

0.281 0.134

0.248 0.132

0.214 0.138

0.36 0.42

0.281 0.158 0.139 0.036 0.92 0.60 0.65 0.57 0.92 0.57 0.70 0.37 0.284 0.238 0.135 0.111 0.99 0.60 1.032

cm3·g−1

pore vol.

Table 1. Adsorption of Cr(VI) by Using Organically Functionalized Silica Materials

1.026 1.38 1.326 0.1 0.1 0.2 0.25

0.80

0.60

0.35

1.09 0.276

0.456 0.794 0.993

0.35

0.812 1.54 1.81

mmol·g−1

adsorption capacity (qe)

2.0

3−4 3.0 6.0

6.2

2.0 3.0 2−3

3.0

3−4

9.0

2.0

2.0

3−4

pH of the Cr(VI) solution

0.74

1.326

0.91 1.10

2.12

0.523

1.875 0.888 0.537

mmol·g−1

qm

0.044

0.229

0.811

0.004

0.0025 0.0035 0.0011

L·mg−1

b R2

0.986

0.9980

0.9988

0.99

0.9870 0.9700 0.9870

Langmuir isotherm

0.62

8.00

0.84 51.4

36.24

7.374 4.487 1.120

mg·g−1

K

1.45

1.62

0.30 6.17

1.70

1.83 2.26 1.53

n

0.94

0.953

0.9988 0.9414

0.9515

0.9870 0.9910 0.9930

R2

Freundlich isotherm

79

91 78

80

90

91 88

81

75

76

86

84

83

76, 77

ref

Journal of Chemical & Engineering Data Review


E

a

calcination amorphous SG MP AAPTS

MPTMS and ED3A neat porous silica grafted-DABCO sol−gel-DABCO CTAB CTAB

neat porous silica AAPTS imidazole MPTMS and EnTMOS

modified by

0.79

MP-SG

GR-DABCO SG-DABCO C-SGC C-SGC H1 H11

0.41 0.62

4.0

NN-silica SOL-IPS-F22 EnTMOS-SH/H+ EnTMOS-SH/ H+reactivated SiO2−SH/ED3A

2.73

7.647 7.208 6.47 5.53

9.1 5.2 4.0

310 260 460

427 453 427 302

7.0

5.11 4.40 1.4 6.6 5.0

nm

pore size

425.2

444 54.5 0.44 155.22 146.36

m2·g−1

mmol·g−1 adsorbent name

surface area

loading of ligand (L0)a

0.66 0.41

0.76 0.71 0.42

0.71 0.090 0.11 0.31 0.26

cm3·g−1

pore vol.

L0 = millimoles of ligand per gram of functionalized silica [i.e., (%N × 10)/N atomic weight].

fumed silica

amorphous silica

silica gelatin

types of silica

Table 1. continued

0.1 0.683

0.181 0.35

0.301

0.556 0.345

mmol·g−1


9.0 3−4

5.8 7.5 4.0

6.0

1−3

4.5 2.5 4.5


0.191 0.193

1.228 1.535

2.28 0.704

mmol·g−1

qm

0.0917 0.0988

0.00372 0.00375

1.025

L·mg−1

b R2

0.9840 0.9890

0.9913 0.9919

0.9988

Langmuir isotherm

2.097 2.716 5.8 ± 1 0.17 2.204 2.414

mg·g−1

K

2.119 2.139 4.90 2.87 1.49 1.53

n

0.9586 0.9811 0.9988 0.9988 0.9900 0.9930

R2

Freundlich isotherm

80

86

92 93 94

96

89

85 87

82

ref




Review

Table 2. Pseudo-First- and Second-Order Kinetics Followed by Silica Based Materials for Cr(VI) Adsorption pseudo−first−order kinetic qe type of silica SBA-15

modified by APTES imidazole triazole calcination

AP-SBA-15 Im-SBA-15 tri-SBA-15 H1 H11

BaTiO3

BaTiO3 @SBA-15

silica gelatin

SBA-15 silica gel

neat silica porous silica

porous silica

adsorbent name

AFC PTFR PANI DMAEMA yerba mate chitosan Cyphos IL [A336][C272] Aliquat 336

mmol·g

pseudo−second−order kinetic

K1 −1

min

−1

qe R

2

Organically Modified Silica Materials 0.089 0.027 0.8680 0.136 0.047 0.9660 0.045 0.017 0.8360 0.127 0.005 0.644 0.130 0.0048 0.670 Inorganically Modified Silica Materials

Polymeric Modified Silica Materials AFC coated silica 0.054 0.8600 PTFR coated silica 0.068 0.0130 0.923 PANI/SiO2 0.091 0.8550 SS-g-DMAEMA 0.096 4.82 0.9680 Biologically Modified Silica Materials Si-Pol 4.92 chitosan coated silica 0.01 0.9700 Ionic Liquids Modified Silica Materials SG-2 0.123 0.243 0.9170 SG-5 0.106 0.155 0.8700 SG-3 0.100 0.033 0.9680

K2 min−1

R2

ref

1.08 2.21 0.55 0.0011 0.0011

0.9990 0.9930 0.9980 0.9980 0.9980

83

0.0051

0.9990

115

0.345 0.402 0.088

0.04 0.0060 0.0025 0.04

1.0000 0.9999 0.9990 0.9990

127 128 131 139

1.325

0.02 0.0002

0.9900

165 163

0.128 0.114 0.111

0.029 0.019 0.004

0.9920 0.9900 0.9920

mmol·g 0.331 0.164 0.121 0.183 0.186

−1

94

182

adsorbent containing a mercapto group, 3-mercaptopropyltrimethoxysilane, and bis[3-(trimethoxysilyl) propyl]ethylenediamine have found to be shown better Cr(VI) removal (EnTMOS-SH, 0.556 mmol·g−1). It is because the adsorbent played a significant role in reduction and sorption of Cr(VI).87 It is important to know that the surfactants have played an important role as structure-directing agents for synthesizing and functionalizing mesoporous silicas, as they are not only found to alter the porosity of silica but also increase the electrostatic interaction with the anionic analytes. The increased interaction resulted in the enhancement of adsorption capacity of Cr(VI). For instance, bifunctionalized adsorbents MCM-41-SH/SO3H and SiO2−SH/ED3A were synthesized in the presence of surfactant CTAB for the treatment of Cr(VI). The thiol group in these adsorbents have able to reduce Cr(VI) into less toxic Cr(III) species which is followed by fast sorption with the SO3H and ED3A groups at pH 1−3 (Table 1).88,89 Showkat and co-workers functionalized mesoporous silica, MCM-41 with N-[3-(trimethoxysilyl)-propyl]aniline by using surfactants, such as CTAB, DTAB, and ETAB (cetyl, dodecyl, or eicosane trimethylammonium bromide). Correspondingly, the adsorbents N-MCM-41-C, N-MCM-41-D, and N-MCM-41-E were generated.90 The adsorption capacities of these adsorbents were analyzed on the basis of the N-content as determined by CHN analysis and the relative decrease in pore volume. The adsorption capacity of N-MCM-41-E (0.80 mmol·g−1) for Cr(VI) was 1.33 times greater than the N-MCM-41-D and 2.3 times greater than the N-MCM-41-C. Anbia and co-workers used surfactant CTAB to synthesize MCM-41 and MCM-48 and compared the adsorption capacities with each other for Cr(VI) removal.91 Table 1 shows that higher content of CTAB in MCM-48 resulted in the better removal of Cr(VI) than the lower content in MCM-41. Also, it confirms the monolayer adsorption of analytes indicating fitting of Langmuir adsorption isotherm for MCM48. Venditti and co-workers also studied the feasibility of

parameters like pH, adsorption data, and kinetic experiments, their efficiency was compared with each other, and it was observed that the AP-SBA-15 has displayed the largest adsorption capacity of 1.875 mmol·g−1. It was attributed to the less stearic hindrance of amino group in aminopropyl moieties than the triazole and imidazole. The adsorption isotherm data was found to be very relevant to both Langmuir and Freundlich models as shown in Table 1. The kinetic experiments suggest that pseudo-second-order model can express adsorption kinetic of Cr(VI) on modified SBA-15 (as given in Table 2). A similar investigation involved sol−gel synthesis method for the development of imidazole functionalized adsorbent (SOL-IPS-F22) by using neat porous silica.85 Surprisingly, the adsorbent SOL-IPS-F22 has shown higher adsorption capacity of 0.704 mmol·g−1 than the other imidazole adsorbents.83,84 For this adsorbent,85 Langmuir adsorption isotherm gave the best fit of the adsorption data as given in Table 1. The most attractive feature of SOL-IPS-F22 is its ability to reuse for 20 cycles of adsorption and desorption process without losing its Cr(VI) removal efficiency. The utilization of aromatic amines was also observed when 2mercaptopyridine (MP) was functionalized on the mesoporous silica SBA-15 and amorphous silica (SG) by the homogeneous synthesis route. The route involved the reaction of 2mercaptopyridine with 3-chloropropyltriethoxysilane prior to immobilization on the support. The Cr(VI) removal efficiency was compared with the MP-SBA-15 and MP-SG. The results have shown maximum adsorption capacity of 0.35 mmol·g−1 for MP-SBA-15 than for MP-SG (0.1 mmol·g−1).86 The reason behind the low adsorption efficiency of MP-SG for Cr(VI) is its nonuniform pore channels which are responsible for the lowering in complexation with the binding site. However, it is observed that, among all the SBA-15 adsorbents (AP-SBA-15, Tri-SBA-15, and Im-SBA-15) reported in Table 1, the adsorbent MP-SBA-15 has shown lowest adsorption capacity. It outlines that the presence of the S atom may affect the adsorption capacity of Cr(VI). However, a bifunctionalized F



Review

Figure 3. A schematic illustration of inorganically modified silica materials showing (a) coordination of FeCl3 ions on the amine-functionalized silica material to form Fe3+−NH2−MSN adsorbent and (b) the exchange of chloride ions with the chromate ions to be removed from wastewater.

ions in the presence of acidic pH (by losing the lone pairs of nitrogen atoms) where they will be electrostatically interacted with the chromium oxyanions. Moreover, the tertiary aliphatic amines when incorporated with silica have shown better results for the removal of Cr(VI) from aqueous solution than the other ones. The modification of silica by using post-grafting method was found to be better than the presynthesis or direct synthesis procedures and, thus, gained some fair adsorbents.

utilizing CTAB−silica gelatin composite (C-SGC) for the removal of Cr(VI) from aqueous solutions in the presence of sulfate ions.92,93 The pH of the solutions was maintained at 5.8 and 7.5. It was observed that the chromate ions were effectively adsorbed from aqueous solution regardless of the presence of competing anions. The same was confirmed by the fitting of adsorption data using Freundlich adsorption isotherm as given in Table 1. Recently, Samjeet and co-worker synthesized a new hybrid material from gelatin and silica via a two-stage sol−gel method by using TEOS as a coupling agent for removal of Cr(VI) from aqueous solution.94 The adsorption capacity of the material was estimated before and after the calcination process and named as H1 and H11. As shown in Table 1, the adsorption was found to be increased due to calcination. The adsorbent has followed Freundlich isotherm and a second-order kinetic model for separation of Cr(VI) (Table 2). Besides the applications of aliphatic and aromatic amino silanes, another group with a bicycle amino compound was utilized for the silica surface modification. 95 A 1,4diazobycycle[2.2.2]octane (DABCO) was immobilized on the silica by grafting and sol−gel methods to prepare GR-DABCO and SG-DABCO adsorbents. These adsorbents have a positive charged group that acts as an anion exchanger for Cr(VI) ions.96 The adsorption capacities of these adsorbents was designed by statistical method to reduce the number of experiments in measuring the highest adsorption for Cr(VI). The effect on the structural parameters of silica after immobilization of DABCO is given in Table 1. After achieving the best conditions for Cr(VI) adsorption, an isotherm was obtained, which was fitted to both nonlinear Langmuir and Freundlich isotherm models. To summarize among the organic groups, amino silanes are found to be the most preferable organosilanes, since the amine groups can be converted into positively charged ammonium

4. INORGANICALLY MODIFIED SILICA MATERIALS Inorganic compounds such as metals and their oxides have played a significant role for the remediation of Cr(VI). Among the metals, iron, especially zerovalent iron (ZVI or Fe0), has proved to be fine material for the removal of Cr(VI).97 However, independently, ZVIs suffer from several problems like high operation, high maintenance costs, agglomeration, and lack of performance, when affected by the reaction precipitates.98 Therefore, they were incorporated with various silicate minerals such as clays, zeolites, and quartz grains to get rid of these drawbacks.99−101 Even these modifications have shown surface deactivation of iron by contaminating its surface and, thus, turned into a barrier.102 Hence, it was discovered that silica could be an alternative for all these compounds. The inorganic compounds have different physical and chemical properties, and, their affinity, reactivity, and reaction rates get increased on modification with silica. In other words, they get captivated by the high surface area, high pore volumes, and pore channels of silica. Figure 3 shows a diagrammatic representation of inorganically modified mesoporous silica MCM-41, which is grafted by the aminopropyl and in the next step activated with the Fe3+ ions. Consequently, it became a suitable adsorbent for the removal of the Cr(VI) ions from wastewater. G


H

LaCl3·7H2O sodium aluminate

neat SBA-15 BaTiO3

SBA-15

APTMS-FeCl3 on neat MCM41 neat MCM-41 AAPTS AAPTS-FeCl3 on Neat MCM41 neat MCM-41 titania

FeCl3·6H2O, FeCl2·4H2O APTMS -FeCl3 on magMCM41

0.23

4.00

3.01

1.97

0.99

0.55

1.2

1.2

BaTiO3@SBA15

(5) TiO2MCM-41 (10) TiO2MCM-41 (15) TiO2MCM-41 (20) TiO2MCM-41 LaSiCS γ-AlOOH spheres

NN-MCM- 41 Fe/NN-MCM41

NH2- MCM41 magMCM-41 Fe3+magMCM41 Fe3+-MCM-41

mesoporous γ-Fe2O3

FeCl3·6H2O, FeSO4·7H2O

neat MCM-41 APTMS

Fe/silica SF-Fe0 Fe@SiO2

mmol·g−1

adsorbent name

Fe(0) anhydrous FeCl3 Anhydrous FeCl3

modified by

neat silica

MCM-41

MCM-41

MCM-41

silica silica fumes silica core/ shell silica KIT-6

types of silica

loading of inorganic modifier

771 185

11.28 9.4

2.00

838 119.29 152.0

2.10

2.53

2.71 2.68

2.90 2.63 2.24

2.80

3.20 2.78

3.17 2.86

4.0

nm

pore size

854

921

1028 938

1283 586 310

500

800 590

1000 750

88.0

472

m2·g−1

surface area

0.905 0.224

0.76

0.89

1.06

1.28 1.14

1.05 0.50 0.25

0.70 0.32

0.87 0.44

cm3·g−1

pore vol.

Table 3. Adsorption of Cr(VI) by Using Inorganically Functionalized Silica Materials

0.335

2.32

1.76

1.11

0.74

2.0

2.0

2.0

0.134

0.067 0.394

mmol·g−1


4.0

4.0 3.0

5.5

6.0

6.0

2.5

6.5−7.5 5.2 4.0


0.118

0.99 0.99

mmol·g−1

qm

0.574

0.32

0.33

0.31

0.26

L·mg−1

b R2

0.98

Langmuir isotherm

4.94

3.21

4.14

5.39

5.88

1.3

1.71

mg·g−1

K

3.98

1.94

1.92

1.79

1.58

0.19

0.14

n

0.88

0.9400

0.9900

R2

Freundlich isotherm

ref

115

116 117

114

113

111, 112

110

103 104 105




Review

isotherm of Cr(VI) on the TiO2-MCM-41 was well-fitted with both the Langmuir and Freundlich models. Lately, an amorphous gel of barium titanate (BaTiO3) was immobilized in a hexagonal mesoporous SBA-15 via an aerosol assisted solid-state reaction to develop BaTiO3@SBA-15 adsorbent.115 The authors reported that the positive charge on the surface of the adsorbent due to the oxygen vacancy leads to the significant adsorption of negatively charged chromate ions (98.2 wt % Cr(VI) removal at pH 4.0). The use of lanthanum encapsulated silica gel/chitosan hybrid composite (LaSiCS) was also found to be extremely attractive for the removal of Cr(VI) from the contaminated water.116 It is reported that the application of LaSiCS composite possessed higher adsorption capacity than SiCS composite, Si, and CS. The optimum pH for the removal of Cr(VI) ions was reported to be 4, and the nature of adsorption was found to be spontaneous and endothermic. The adsorption isotherm of the adsorbent for Cr(VI) removal was well-fitted to Langmuir model as depicted in Table 3. The concept of using all of these substances in a single composite is directly indicated by their capacities of having adsorption sites, which are meant for the chromate sorption. Other than the transition metal ions and their oxides, some light metal oxides such as boehmite (γAlOOH) have been also used for silica colloidal spheres for the development of novel adsorbents. Here, silica is not only served as a template but also led to the hydrolysis of aluminate anions (AlO2−) at high temperature, which resulted in the formation of the lamellar γ-AlOOH around the silica spheres. It is collectively called as hierarchical SiO2@γ-AlOOH core−shell structure.117 Overall, it is observed that the transition metal ions and compounds, with their enhanced morphologies (nanosize particles), when modified with silica, their stability and longevity increased as an adsorbent. Especially, iron metal and their oxides have achieved magnificent adsorption capacities for the chromium ions when modified with different forms of silica. It is because silica provides them solid support as well as prevents them from the agglomeration. Other metal oxides and compounds such as titanium oxides, lanthanum chloride, or zirconium oxides have also shown considerable adsorption capacities toward the Cr(VI) from aqueous solutions.

Oh et al. investigated that silica enhances the longevity and activity of Fe0 by providing surfaces to the reduction of Cr(VI) and, preferentially, adsorb reaction products (Cr−Fe hydroxides).103 The rate-enhancing effect was observed mainly due to the higher surface area of silica and its greater affinity to the reaction products (Table 3). In the case of silica dose, the Cr(VI) removal increases with an increase in the dose of silica, and the appropriate mass ratio for Fe0/silica was found to be around 1:2. Recently, silica fumes (SF, a nontoxic and commercially available) supported Fe0 NPs (nanoparticles) were synthesized by borohydride reduction of FeCl3.104 The adsorbent was used to stabilize the Cr(VI) ion removal from the aqueous solutions by conducting batch experiments. It was observed that 0.4 g·L−1 of Fe0 concentration in silica fumes was responsible for 88% removal of Cr(VI) as given in Table 3, whereas unsupported Fe0 removed only 65.45% of Cr(VI) which has proved higher removal capacity of SF−Fe0 than the unsupported Fe0. Li and co-workers synthesized SiO2-coated Fe nanocomposites (Fe@SiO2) and used for the degradation of Cr(VI).105 They demonstrated that the degradation ability of Fe@SiO2 was 83.64% higher than their uncoated counterparts. A removal of 0.394 mmol·g−1 for Cr(VI) was achieved from the aqueous stream (Table 3). Apart from the zerovalent iron (ZVI) nanoparticles, magnetic iron oxide nanoparticles (Fe3O4, Fe2O3) were also found to be utilized for the remediation of heavy metals because of their large surface area.106−108 However, due to their small sizes (nm), their response to the magnetic field reduces, which has resulted in the low desorption power of them for the analytes.109 In view of this, mesoporous magnetic Fe2O3 were synthesized, although, their particle sizes are much larger (in μm) than the magnetic iron oxide NPs (in nm). However, their response to an external magnetic field was far better than those iron oxide NPs, which resulted in the satisfactory remediation of heavy metals. One of the examples is γ-Fe2O3 modified mesoporous silica KIT-6 which was synthesized by using green methodology. Table 3 shows an adsorption capacity of 0.134 mmol·g−1 for Cr(VI) at the acidic condition (pH 2.5). The desorption was also carried out using 0.01 M NaOH, which resulted in the almost complete recovery of Cr(VI), and the adsorbent was reused at least five times.110 In similar investigations, mesoporous silica MCM-41 with aminopropyl and iron oxide NPs were used for the synthesis of magnetic MCM-41 by following two-step synthesis. The products Fe3+magMCM-41 and Fe3+-MCM-41 showed high surface area as well as high magnetization, as given in Table 3. These modifications were found to be responsible for the selective adsorption of CrO42− ions from single components, as well as binary components.111−113 Besides the use of iron oxide compounds, another metal oxide such as titanium dioxide was also considered for the immobilization on mesoporous silica by postsynthesis method. Parida et al. immobilized TiO2 by loading different weight percentages of titania on the mesoporous MCM-41.114 It was observed that the surface area and pore diameter of MCM-41 were decreased after increase in TiO2 loading (Table 3) that could be to the pore blocking. Interestingly, TiO2 and neat MCM-41 exhibited very less adsorption of Cr(VI) as compared to the TiO2-MCM-41. Actually, the surface of TiO2-MCM-41 was equipped with the positive charges and thus, the adsorption of the chromate ions was according to the amount of titania loaded on MCM-41. The highest adsorption capacity for Cr(VI) was found as 2.32 mmol·g−1 at pH 5.5. The adsorption

5. POLYMER-MODIFIED SILICA MATERIALS Polymers have played a vital role for the remediation of toxic metal ions, especially chromium, when they were combined with the various adsorbents.118−123 They are the repeated units of organic components (monomers) having key functional groups which are meant to have attractive chemical properties. Once the chemical properties of polymers were incorporated into the properties of silica (high porosity, large surface area, high surface silanols, water, and thermal stability), the modified silicas will serve as promising materials for the removal of Cr(VI) ions. Among the various forms of silica, silica gel has been widely used to be modified with polymers for the removal of toxic metal ions.124−126 Silica gel provides a substantial support for the polymers so that the functional groups of the polymer can be readily available for the metal ions to interact. A noteworthy example of this type is resinous aniline formaldehyde condensate (AFC) polymer, synthesized by polymerization of aniline in acidic medium with formaldehyde, and further was coated on the silica gel to from a granular adsorbent. Later, it is used for the removal of Cr(VI) from I



Review

Table 4. Adsorption of Cr(VI) by Using Polymer-Functionalized Silica Materials Langmuir isotherm surface area type of silica silica gel

modified by AFC PVP PTFR PANI PEI PPY NPs PANI DMAEMA

neat silica

DMAEMA calix[4]arenes calix[4]arenes with Py units PEI AHIBA PAA

adsorbent name

m2·g−1

pore size

nm

pore vol.

cm3·g−1

AFC coated silica gel PTFR coated silica PANI/SiO2 QPEI/SiO2 PPYNPg-FSG PANIPG-SSG DMAEMA grafted silica SS-g -DMAEMA

adsorption capacity (qe) pH of the Cr(VI) solution

mmol·g−1 0.146

3.0

128

0.334

5.0 1.0

235

0.545 1.233 0.327

2.0 3.0 2.0

720

1.164

2.0−3.0

388.7

9.5

0.92

20.6 310.4

0.39

qm

b

K

mmol·g−1

L·mg−1

R2

mg·g−1

n

R2

ref

0.005

0.9800

2.39

1.56

0.9900

127

0.345

0.097

0.9880

1.15

1.527

0.9960

130 128

0.546 1.31

0.125 0.467

0.9960 0.9990

12.93

2.417

0.9990

131 132 133 134

2.5−5.0

0.586

0.024

0.9920

138

1.5−5.4

0.447

0.79

0.9990

139

0.087

1.5

144 145

0.26 0.498

6.2 10.0 3.0

147 148 149

calixCECHES PEI-silica MOXC-2

Freundlich isotherm

aqueous streams and has shown 0.146 mmol·g−1 of adsorption capacity, as depicted in Table 4.127 Recently, in continuation, our group has prepared a new adsorbent by coating of ptoluidine formaldehyde resin (PTFR) on silica gel and applied for the removal of Cr(VI).128 As shown in Figure 4, the activity of the developed adsorbent was dependent on the pH of the solution. It is reported that the positive inductive effect of the methyl group at para-position of the amine enhances electrostatic interaction with chromate ions. Subsequently, the

stronger interaction leads to the maximum removal of Cr(VI) (0.345 mmol·g−1). The adsorption was well fitted with Langmuir and Freundlich isotherm (as shown in Table 4) and has followed second order kinetics (Table 2). Later, the adsorbent was regenerated with 0.2N NaOH solution, and complete recovery of Cr(VI) was achieved. The other reported polymer that is coated with silica gel for Cr(VI) removal is a weak base poly(4-vinylpyridine) (PVP)129,130 and polyaniline (PANI).131 The adsorption capacities and kinetics are shown in Tables 4 and 2, respectively. Apart from the physical modification of silica gel by polymers, the chemical modification of silica gel has also been carried out. For instance, grafting of the silica surface by polyethylenimine (PEI) in which the silica surface was initially modified by γ-chloropropyltrimethoxysilane (CPTMS) and further processed by the couple grafting of PEI on SiO2. For the grafted PEI, two polymeric reactions, tertiary amination and quaternization, were performed, which leads to the formation of composite particle QPEI/SiO2.132 This polyelectrolyte molecule produces quaternary ammonium cations in aqueous solution, which leads to the electrostatic attraction with the chromate anions. The adsorption capacity of QPEI/SiO2 was entirely dependent on the quaternization of grafted PEI. It is because the adsorption of Cr(VI) was found to be increased (1.31 mmol·g−1, Table 4) with an increase in the extent of quaternization. It is in turn responsible for the rise in the number of quaternary ammonium cations on the QPEI/SiO2 surface. The same “grafting” method was considered when the silica gel was modified by using polypyrrole nanoparticles to synthesize the adsorbent. Herein, initially, the silanol groups of the silica surface were functionalized with dimethyldichlorosilane (DDS). Later, it is grafted by using polypyrrole nanoparticles to give polypyrrole-NPs-grafted-functionalizedsilica-gel (PPYNP-g-FSG) with a high surface area (Table 4).133

Figure 4. A diagrammatic representation of the polymer-modified silica material where (a) p-toluidine formaldehyde resin (PTFR), and (b) its coating on the silica gel to form a granular adsorbent, was applied (c) to remove chromate ions from aqueous solution. J



Review

Figure 5. A schematic diagram of the biologically modified silica material depicting (a) the attachment of biopolymer chitosan with the amine-grafted mesoporous silica in the presence of formaldehyde, (b) to form chitosan-modified-MSN adsorbent, and (c) the Cr(VI) ions get attached electrostatically with the protonated amine of the chitosan.

for creating hydrogen bonding with dichromate ions (HCr2O7−) at lower pH and, thus, leads to the adsorption of Cr(VI) ions up to 87% (0.087 mmol·g−1, Table 4).144 It is reported that, among these polymers, the high adsorption was shown by the compound that contained tert-butyl groups in their cone conformation. These results suggest that the rigid amide moiety at the lower rim of caliex[4]arene plays a significant role in the host−guest interactions. Due to higher oxidative stability of amides, these polymers are suitable candidates for the removal of chromate ions. Recently, the same monomer calix[4]arene is reported for the adsorption of dichromate anions. For which the adsorbent was prepared by using calyx[4]arene containing pyridinium units with silicabased polymer (ECHES).145 The partially protonated pyridyl groups of the calix[4]arenes-based polymers were found to be responsible for making hydrogen bonds with dichromate ions at pH 1.5 (Table 4). It is interesting to note that the polymeric/inorganic hybrid nanoparticles can also be prepared as adsorbents for the removal of potentially toxic metals.146 A biomimetic synthesis of hybrid silica/dendrimeric polymer nanosphere is performed by using silicic acid and nonsymmetrical analogues of hyperbranched polymer poly(ethylene imine), PEI.147 The formation of siloxanes bridges between the polymer and the silica makes the polymer insoluble in water and retains the adsorption capacity of PEI-silica. More than 90% adsorption of dichromate anions has been reported with the protonated amine groups of the PEI within 1 h. These results may be attributed to the formation of conventional metal−ligand and charge-transfer complexes. The sol−gel processing of mixed metal oxides has some advantages such as high reactivity of the adsorbent and convenient preparation method. For example, the mesoporous SiO2/Al2O3 mixed oxide was prepared via the sol−gel process. The active functional group was achieved on the material by the addition of the carboxylate groups such as alpha-hydroxyl isobutyric acid (AHIBA) during the gelation process and further calcined to obtain SiO2−Al2O3-AHIBA composite powder (i.e., MOXC). Here, the silica content provides high surface area, whereas the aluminum oxide forms the cross-

However, the adsorption capacity of PPYNP-g-FSG (0.33 mmol·g−1) was found to be less than the QPEI/SiO2 (1.23 mmol·g−1 ). In a continuation of postgrafting method, Chowdhary and co-workers initially silanized the silica gel by 5% ethanol solution of dimethyl dichlorosilane (DDS). They further grafted the prepared polyaniline nanoparticles on silanized silica gel to develop PANINP-G-SSG adsorbent. The surface area of the adsorbent was found to be 720 m2 g−1, and the adsorption capacity was 1.16 mmol·g−1 of Cr(VI), as given in Table 4.134 Another method called “radiation-induced grafting”, in which polymerization of the monomers was initiated by using electromagnetic radiations, has been widely utilized to prepare silica based adsorbents for remediation of heavy metal ions.135−137 A DMAEMA (dimethyl aminoethyl methacrylate) grafted silica is made by initial silanization of silica gel with trimethylchlorosilane and later grafting with DMAEMA by using γ-rays. The DMAEMA grafted silica was further protonated in acidic medium and used as an adsorbent for the treatment of Cr(VI). This adsorbent gave maximum adsorption capacity of 0.586 mmol·g−1 (Table 4).138 The silanization of silica was necessary as it generated free radicals (−CH2•), which were responsible for the high grafting yield (GY). The high grafting yield in turn produces amino group formation on the silica surface that gets protonated at lower pH (2.5 to 5.0). Then, it leads to high ion exchange capacity (IEC) by means of electrostatic attraction with chromate anions. The same group used electron beam (EB) for the irradiation of silanized silica and performed Cr(VI) removal from the aqueous stream.139 Both the adsorbents have followed Langmuir adsorption isotherm model (as depicted in Table 4) and observed pseudo-first- and second-order kinetics (Table 2). Silica gel because of its high surface area and chemical stability is quite capable of being immobilized even with the supramolecules, especially calix[4]arene polymer.140−142 For example, calix[4]arene amido derivatives were modified by using 3-aminopropyl triethoxysilane (APTES) to make a polymer and further immobilized on silica gel.143 These polymers containing amide functional groups were responsible K


Journal of Chemical & Engineering Data 164 165 167 0.42 0.47 0.9970 0.54 0.033 0.168 0.040 1.0 3.0 2.0

0.224

0.168

ref

163 1.0000 1.182 12.87 1.0000 0.04 2.53 3.0

0.9460 0.8960 0.35 0.37 0.30 0.398

4.0

0.305 0.4

1.06 5.03

0.9720 0.9840

3.48 ± 0.35 2.71 ± 0.36

R2 n mg·g−1 L·mg−1 mmol·g−1 cm3·g−1

160.8

silica gel

cellulose acetate yerba mate Scenedesmus obliquus

chitosan coated silica FCA/SiO2 Si-Pol porous silica

silicate−chitosan−silicate silicate−chitosan−silicate− chitosan−silicate chitosan

SCS SCSCS

126.49

nm m2·g−1 adsorbent name modified by type of silica

adsorption capacity (qe) pore vol. pore size surface area

Table 5. Adsorption of Cr(VI) by Using Biologically Modified Silica Materials L

mmol·g−1


qm

b

R2

K

Freundlich isotherm Langmuir isotherm

6. BIOLOGICALLY MODIFIED SILICA MATERIALS Biologically available adsorbents, biosorbents, have proved to be a good alternative for the treatment of various toxic metal ions and Cr(VI), because of their low cost and significant uptake capacity.150−152 Whether living or nonliving, their immobilization on the appropriate matrix surface leads to an enhancement of their adsorption capacity because of the improved physical and mechanical properties, for example, mechanical strength, larger pore volumes, and rigidity on the solid.153−156 One of the suitable porous solid supports for the biosorbents is silica that has been used extensively for the removal of toxic metal ions.157−160 Both living and nonliving materials are employed in the preparation of silica-based biosorbents for the removal of Cr(VI). Biopolymers such as chitosan (glucosamine biopolymer) have played a significant role in this particular application when immobilized on a solid support. It not only retains its active surface phenomena but also enhances the adsorption capacity.161 Figure 5 demonstrates the same. The adsorption process was successful because of the electrostatic interaction between the protonated amines of the chitosans and the negatively charged chromate anions. In another case, the generation of layer-by-layer silicate−chitosan composite biosorbent was prepared by using dual mode of silica-chitosan composite films on the glass surface. For instance, silicate− chitosan−silicate (SCS) and silicate−chitosan−silicate−chitosan−silicate (SCSCS) have shown preferable results for the adsorption of Cr(VI). It is because the silicate layers in the composite preserves the leakage of the polysaccharides of chitosan. Due to this, the surface of chitosan remains active which resulted in the protonation of the amine group present at the surface, at acidic pH. Consequently, it leads to the increased electrostatic interaction with chromium anions.162 However, more layers of silicate in SCSCS composite film has shown better results in comparison with SCS composite as indicated in Table 5. The equilibrium time was achieved within 4 h for SCSCS composite while, it took 8h for SCS. The equilibrium data was well-fitted with the Langmuir and Freundlich isotherms. In another investigation, a modification of silica is

neat silica

linking with carboxylate groups of AHIBA so that better adsorption sites would be available for the chromium ions by the polymeric oxide (Table 4).148 Recently, acrylamide was polymerized within the pore regions of the silica particles. The composite particle maintained the mechanical properties of polyacrylamide (PAA) to serve efficiently in the adsorption process. The adsorption performance for Cr(VI) was observed at acidic pH 3.0 (0.498 mmol·g−1, Table 4), and the parameters like initial chromium concentration and the amount of the adsorbent used have been studied.149 It is remarkable that the polymers have played a prominent role as an adsorbent for the adsorption of Cr(VI), when modified with silica either by physical or chemical processes. From the chemical point of view, there were several methods of chemical modifications considered, such as grafting, radiationinduced grafting, biosilification, and the gelation process to form polymer-modified silica. Supramolecules and dendrimers are also used in their modified forms with silica for Cr(VI) removal from aqueous solution. However, polymers are also organic compounds, but their highly branched chains with various functional groups present led them to adsorb selective metal ions, which distinguish them from the simple organic compounds.

162

Review



Review

Table 6. Adsorption of Cr(VI) by Using Ionic Liquid Modified Silica Materials Langmuir isotherm surface area type of silica

modified by

porous silica SBA-15

Vitamin B4 MTICl

porous silica

Cyphos IL 104 [A336] [C272] Aliquat 336

adsorbent name

m2·g−1

pore size

nm

pore vol.

cm3·g−1

Si-NH2-VB4


mmol·g−1


0.442

3.0

SBA15Im0.10Cl1h SBA15Im0.15Cl1h SBA15Im0.20Cl1h SBA15Im0.25Cl3h SG-2

716 773 697 418 343.9

6.50 6.30 6.20 5.80 3.32

1.17 1.19 1.08 0.606 0.313

0.165

SG-5

611.5

3.27

0.509

0.131

SG-3

4.6

2.0

0.095

5.2

qm

b

mmol·g−1

L·mg−1

Freundlich isotherm

K

R2

mg·g−1

n

R2

ref 180

0.98 1.21 1.43 1.74 0.166

1.5 1.3 1.5 1.1 2.19

0.9910 0.9950 0.9950 0.9960 0.9990

181

13.0

10.3

0.8520

0.133

1.13

0.9920

8.04

6.46

0.8970

0.105

0.13

0.9920

3.95

4.30

0.8770

182

also been investigated on the adsorption capacity of these adsorbents. Overall, it is observed that the biosorbents whether living or nonliving, have been utilized for the removal of Cr(VI), when incorporated into silica. Living cells comprised of bacterial and algal cells whereas nonliving contains chitosan, yerba mate, and cellulose acetate. It is noticeable that the polymeric protein chitosan has shown a maximum adsorption capacity toward Cr(VI) because of the amine groups present in them.

further reported when the chitosan gel in 2% acetic acid was coated on silica on a solid support.163 Here, the reaction was processed in acidic solution (pH 3.0), and the same technique was carried out for the adsorption of Cr(VI). The surface area of the chitosan-coated silica was found to be 160.8 m2·g−1. The equilibrium time was reached within 180 min of the reaction, and the data was well-fit to the Langmuir and Freundlich isotherm models with maximum adsorption capacity of 2.53 mmol·g−1 (Table 5). The kinetic study revealed that the adsorption of Cr(VI) has followed a second order kinetics rather than pseudo-first-order, as given in Table 2. It is interesting to note that many other biopolymers have also been utilized for the removal of Cr(VI). They are immobilized on the silica either by chemical modification or cross-linking. A polymer cellulose acetate (CA), get actively interacted by the electrostatic mean with the silanol groups of the silica that was already modified by the coupling agent 3ureidopropyl triethoxysilane. It has resulted in the generation of NH2-functionalized cellulose acetate/silica composite nanofibrous membrane (FCA/SiO2).164 The FCA/SiO2 has shown good Cr(VI) adsorption capacity and is well described by the Langmuir adsorption behavior as depicted in Table 5. Also, it is quite noticeable that the composite membrane has shown five consecutive cycles of adsorption/desorption which made this adsorbent economically viable. Recently, a biopolymer known as Yerba mate (llex paraguariensis), a source of polyphenol is obtained from milling residual dust and was immobilized on the SiO2 matrix by glutaraldehyde cross-linking. This hybrid material has shown a maximum adsorption capacity of 0.04 mmol·g−1 of Cr(VI) at acidic condition (pH 3.0) for 60 min and the equilibrium time was achieved within 50 min.165 Both Langmuir and Freundlich isotherms showed the fitting of the material for Cr(VI) removal (as given in Table 5) and followed pseudo first and second order kinetics (Table 2). Among the living organisms, bacterial and algal cells are frequently immobilized on the silica matrices for the removal of Cr(VI). It is because silica provides them a substantial support which in turn make cells serve a large surface area and contact interface for the metal ion interaction.166 For example, Scenedesmus obliquus and Arthospira maxima cells which after immobilization on the silica gel, has shown excellent adsorption capacity for Cr(VI).167 The influence of other parameters such as pH, contact time, and initial chromium concentration has

7. IONIC LIQUID MODIFIED SILICA MATERIALS Ionic liquids constitute of organic cation and organic or inorganic anion and have melting temperatures below 100 °C. They have attractive physicochemical properties such as low volatility and vapor pressure, high solvent capacity, and hydrophobicity. Also, their high separation efficiency and selectivity make them highly suitable for the separation of potentially toxic metal ions.168−171 The use of solid supports for them not only preferable from the economic point of view, but also enhances their surface activity. Since these solid supports provide them high surface area and high thermal and chemical stability.172−174 Once again, among the solids, silicas are proving to be reasonable materials along with the ionic liquids for the treatment of potentially toxic metals.175−177 Although, for the removal of Cr(VI) ions, the ionic liquid supported on silica gel as an adsorbent is still in progress, and not much work has done earlier in this particular area. Therefore, it is challenging to sum up those experiments that are needed to be revealed, so that it would be beneficial for the researchers to carry out future inventions using these green adsorbents. Preliminary works have been reported on the physical and chemical modifications of silica with ionic liquid.178,179 Recently, a typical quaternary ammonium salt, i.e., (2hydroxyethyl) trimethylammonium chloride, also known as choline chloride [ChCl] or Vitamin B4, was immobilized on the 3-aminoprpyl trimethoxysilane modified silica (Si-NH2) via formation of hydrogen bond.180 The immobilized silica gel phase [Si-NH2−VB4] has achieved a good adsorption capacity toward chromate ions via anion exchange phenomenon. The adsorption process was conducted at various pH conditions and the maximum chromate anions exchange capacity of 0.442 mmol·g−1 was observed at pH 3.0 as given in Table 6. Also, the equilibrium time was achieved within 5 min, and nearly 100% M



Review

Figure 6. A diagrammatic representation of (a) the development of ionic liquid modified silica adsorbent by applying the direct or co-condensation method and (b) the protonated imidazole ring exchanged chloride ions with the chromate ions in the acidic medium.

1 h, 1 h, and 3 h for SG-2, SG-5, and SG-3, respectively. Also, the equilibrium data were well-fitted with the Langmuir isotherm model which is as shown in Table 6. The adsorption kinetics could be well-described by pseudo-second-order kinetic model (Table 2). Overall, it can be concluded that the ionic liquid modified silica materials can be considered as novel adsorbents since they have shown high anion exchange capacity as well as they are eligible to reuse many times. Their high desorption capacity and attractive physicochemical properties makes them to be considered as “green adsorbents”.

adsorption was achieved. The entire process showed that the adsorbent is an eco-friendly alternative for the removal of Cr(VI). Apart from that, the direct synthesis (co-condensation) of functionalized silica was carried out by using ionic liquid, 1methyl-3-(triethoxypropyl) imidazolium chloride [MTICl] with a template-directed hydrolysis polycondensation of TEOS (Figure 6). It gives ordered N-methylimidazolium functionalized mesoporous silica (SBA-15) anion exchanger [SBA15ImXClt]. In which x denote different mole ratios of ionic liquid in the adsorbent and t is the pre hydrolysis time taken by the reagent TEOS to obtain the modified adsorbent.181 The prepared samples with a rod-like morphology showed high surface areas (400 m2 g−1) and good thermal stability (387 °C). It was observed that the mesoporous silica SBA-15 with a molar ratio of 25% MTICl has shown a maximum adsorption capacity of 1.74 mmol·g−1 for Cr(VI). The adsorption reaction was carried at weak acidic condition (pH 4.6) and the equilibrium data was well fitted to the Langmuir isotherm model (as can be seen in Table 6). It was observed that 95% of the desorption of Cr(VI) was achieved when the process was carried out in the basic solution (0.1 mol· L−1 NH3·H2O and 0.5 mol·L−1 NH4Cl). Also, the adsorbent was reused several times. Liu et al. prepared quaternary phosphonium and ammonium ionic liquids modified silica by the sol−gel process and used as adsorbents for the removal of Cr(VI). The ionic liquids that have been utilized for the functionalization of silica were trihexyl (tetradecyl) phosphonium bis-2,4,4-trimethylpentylphosphine (Cyphos IL 104) and trialkylmethylammonium bis2,4,4-trimethylpentylphosphinate ([A336][C272]). These ionic liquid functionalized silica adsorbents (SG-2 and SG-5) were compared with the Aliquat 336 (trialkylmethylammonium chloride) and Cyanex 272 (bis(2,44-trimethylpentyl)phosphonic acid) modified silica sorbents (SG-3 and SG-4). It was found that the maximum adsorption efficiencies of Cr(VI) were obtained in the pH range 0 to 0.2 for SG-2 and SG-5. However, for SG-3, it was achieved in the pH range of 0 to 5.2 as shown in Table 6.182 The equilibrium was achieved at

8. FUTURE PROSPECTS An efficient treatment of Cr(VI) is crucial because of its numerous sources such as the paint industry, textile dyeing, leather tanning, metal finishing, and electroplating. Further, it exists in the solution as various species, which is entirely dependent on the pH and its concentration. Therefore, it has low acute and chronic toxicity to the human beings. As a result, it is still a challenge to bring down the concentration of Cr(VI) below the tolerable limit of 0.05 mg·L−1 from all kinds of aqueous streams. Adsorption of Cr(VI) is one of the most promising method of treatment of Cr(VI) from all such streams. Since, it has got striking advantages such as low energy demand and chemical investment, high removal efficiency, and easy recycling. The importance of using silica-based materials for the treatment of Cr(VI) is extensively investigated by opting batch mode technique. The silicas because of their varied types and physical properties offer an excellent alternative for the preparation of robust adsorbents. Their large pore volume allows natural impregnation or immobilization of selected solvents. Their high surface area including the outer surfaces, mesoporous surfaces, and mesoporous walls are tunable, and their sizes can be varying with the use of various templates and additives. Although, all of the silica-based materials have shown reasonable adsorption; however, the criterion such as N



Review

Notes

adsorption capacity, cost, selectivity, equilibrium time, regeneration, and reusability contains immense importance. On the basis of these parameters, it was found that the organically functionalized silica materials are quite useful and, hence, are extensively studied for Cr(VI) adsorption. Various types of silicas were exploited for their support with aliphatic and aromatic amines. The aliphatic amines due to the less stearic hindrance and good protonating ability have shown the superiority over the aromatic amines. The application of statistical design in order to reduce the number of experiments appears to be quite attractive and, hence, should be frequently examined. Further, extensive investigation is required in the application of structure-directing agents for the synthesis of functionalized silicas. It may be helpful in the development of selective adsorbents. The use of inorganically modified silicas may provide better selectivity, stability, and longevity of the adsorbent. Notably, the role of nanocomposites appears to be highly promising since it combines both adsorption as well as reductive degradation of toxic Cr(VI) to the nontoxic Cr(III) metal. Additionally, the combination of organic and inorganic compounds within silica have shown better selectivity as well as enhanced ion-exchange capabilities. However, the studies are mostly limited to the use of iron metal and their oxides. Several other nanoparticles and composites need to be investigated to test their efficiency for the removal of Cr(VI). The regeneration and reuse of adsorbent should also be investigated thoroughly. The use of long chain polymers may offers distinctive property over the organically modified silica. Various methods are available for their immobilization in silica matrices. Some of the polymers such as calyx[4]arene plays an important role for the host−guest interactions. The interaction may be helpful in obtaining high selectivity for the adsorbent. In view of this, several new polymers should be investigated for the adsorption of Cr(VI). Considering the cost of the compounds and environmental compatibility, biosorbents are preferable materials for the adsorption of Cr(VI). For instance, a biosorbent like chitosan with silica, which is full of amine functional groups, has achieved significant adsorption of Cr(VI). In spite of this, they are not extensively investigated. It was noticeable that the capacity of the ion-exchange was thoroughly seen in ionic liquid immobilized silicas, and they are proved to be advantageous adsorbents. In fact, they are the combination of different organic and inorganic moieties, and because of their magnificent properties, they may be the best source of future investigation. Their high capacity for the ion-exchange mechanism makes them a perfect material for the large uptake of toxic metal ions as well as it allows to reuse them many times. Hence, the application of both biosorbents and ionic liquids for the modification of silica appears to be highly beneficial for the treatment of Cr(VI).

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The authors declare no competing financial interest.

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ABBREVIATIONS AAATPS, N-(3-trimetoxysilylpropyl)diethylenetriamine; AATPS, [1-(2-amino-ethyl)-3-aminopropyl] triethoxysilane; AFC, aniline formaldehyde condensate; Aliquat 336, trialkylmethylammonium chloride; APTES, 3-aminopropyl triethoxysilane; APTMS, 3-aminopropyl trimethoxysilane; [A336][C272], trialkylmethylammonium bis 2,4,4-trimethylpentylphosphinate; CPTES, 3-chloropropyltriethoxysilane; CTAB, cetyltrimethyl ammonium bromide; Cyphos IL, trihexyl (tetradecyl) phosphonium bis 2,4,4-trimethylpentylphosphine; DABCO, 1,4-diazobicyclo [2,2,2] octane; DDS, dimethyl dichlorosilane; DETA, N-(3-trimethoxysilylpropyl) diethylenetriamine; DMAEMA, dimethyl aminoethyl methacrylate; DTAB, dodecyl trimethyl ammonium bromide; ECHES, calix[4]arene containing pyridinium units with silica; ED3A, N-[(3-trimethoxysilyl)-propyl]-ethylenediamine triacetate; EnTMOS, bis-[3-(trimethoxysilyl)propyl] ethylenediamine; ETAB, eicosane trimethyl ammonium bromide; IEC, ion exchange capacity; MCM, mobile composition of matter; MP, 2-mercaptopyridine; MPTMS, mercaptopropyl trimethoxysilane; MTICl, 1-methyl-3-(triethoxypropyl) imidazolium chloride; MTMS, mercaptotrimethoxysilane; NPs, nanoparticles; PANI, polyaniline; PEI, polyethylene imine; PPY, polypyrrole; PTFR, p-toluidine formaldehyde resin; PVP, poly(4-vinylpyridine); SBA, Santa Barbara amorphous; TEOS, tetraethyl orthosilicate; THAM, tris(hydroxymethyl) methylamine; TMPA, N-[3-(trimethoxysilyl)-propyl] aniline; TMSEDA, N[3-(trimethoxysilyl)propyl]-ethylenediamine

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ps_kulkarni@rediffmail.com; [email protected]. Tel. +91 20 24304161. Fax: +91 20 24389509. Funding

The authors gratefully acknowledge the research grant provided by DRDO (ERIP/ER/1003883/M/01/908/2012/D, R8D/ 1416, dated. 28-03-2012), New Delhi, India. One of the authors, Mr. Manish Kumar Dinker, wishes to acknowledge DIAT, Pune, for his Ph.D. fellowship. O



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